<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Miscellaneous Notes</title>
<link rel="stylesheet" href="../../../doc/src/boostbook.css" type="text/css">
<meta name="generator" content="DocBook XSL Stylesheets V1.79.1">
<link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset">
<link rel="up" href="../variant.html" title="Chapter 42. Boost.Variant">
<link rel="prev" href="design.html" title="Design Overview">
<link rel="next" href="refs.html" title="References">
<meta name="viewport" content="width=device-width, initial-scale=1">
</head>
<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
<table cellpadding="2" width="100%"><tr>
<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../boost.png"></td>
<td align="center"><a href="../../../index.html">Home</a></td>
<td align="center"><a href="../../../libs/libraries.htm">Libraries</a></td>
<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td>
<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
<td align="center"><a href="../../../more/index.htm">More</a></td>
</tr></table>
<hr>
<div class="spirit-nav">
<a accesskey="p" href="design.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../variant.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="refs.html"><img src="../../../doc/src/images/next.png" alt="Next"></a>
</div>
<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="variant.misc"></a>Miscellaneous Notes</h2></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="misc.html#variant.versus-any">Boost.Variant vs. Boost.Any</a></span></dt>
<dt><span class="section"><a href="misc.html#id-1.3.43.7.3">Portability</a></span></dt>
<dt><span class="section"><a href="misc.html#variant.troubleshooting">Troubleshooting</a></span></dt>
<dt><span class="section"><a href="misc.html#variant.ack">Acknowledgments</a></span></dt>
</dl></div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="variant.versus-any"></a>Boost.Variant vs. Boost.Any</h3></div></div></div>
<p>As a discriminated union container, the Variant library shares many
    of the same features of the Any library.
    However, since neither library wholly encapsulates the features of the
    other, one library cannot be generally recommended for use over the
    other.</p>
<p>That said, Boost.Variant has several advantages over Boost.Any,
    such as:

    </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">Boost.Variant guarantees the type of its content is one of a
        finite, user-specified set of types.</li>
<li class="listitem">Boost.Variant provides <span class="emphasis"><em>compile-time</em></span>
        checked visitation of its content. (By contrast, the current version
        of Boost.Any provides no visitation mechanism at all; but even if it
        did, it would need to be checked at run-time.)</li>
<li class="listitem">Boost.Variant enables generic visitation of its content.
        (Even if Boost.Any did provide a visitation mechanism, it would enable
        visitation only of explicitly-specified types.)</li>
<li class="listitem">Boost.Variant offers an efficient, stack-based storage scheme
        (avoiding the overhead of dynamic allocation).</li>
</ul></div>
<p>

  </p>
<p>Of course, Boost.Any has several advantages over Boost.Variant,
    such as:

    </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">Boost.Any, as its name implies, allows virtually any type for
        its content, providing great flexibility.</li>
<li class="listitem">Boost.Any provides the no-throw guarantee of exception safety
        for its swap operation.</li>
<li class="listitem">Boost.Any makes little use of template metaprogramming
        techniques (avoiding potentially hard-to-read error messages and
        significant compile-time processor and memory demands).</li>
</ul></div>
<p>

  </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="id-1.3.43.7.3"></a>Portability</h3></div></div></div>
<p>The library aims for 100% ANSI/ISO C++ conformance. However, this is
    strictly impossible due to the inherently non-portable nature of the
    Type Traits library's
    <code class="computeroutput">type_with_alignment</code> facility. In
    practice though, no compilers or platforms have been discovered where this
    reliance on undefined behavior has been an issue.</p>
<p>Additionally, significant effort has been expended to ensure proper
    functioning despite various compiler bugs and other conformance problems.
    To date the library testsuite has
    been compiled and tested successfully on at least the following compilers
    for basic and advanced functionality:

    </p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
<col>
<col>
<col>
</colgroup>
<thead><tr>
<th> </th>
<th>Basic</th>
<th>
              <code class="computeroutput">variant&lt;T&amp;&gt;</code>
            </th>
<th>
              <a class="link" href="tutorial.html#variant.tutorial.over-sequence" title="Using a type sequence to specify bounded types">
                <code class="computeroutput">make_variant_over</code>
              </a>
            </th>
<th>
              <a class="link" href="tutorial.html#variant.tutorial.recursive.recursive-variant" title="Recursive types with make_recursive_variant">
                <code class="computeroutput">make_recursive_variant</code>
              </a>
            </th>
</tr></thead>
<tbody>
<tr>
<td>Borland C++ 5.5.1 and 5.6.4</td>
<td>X</td>
<td>X</td>
<td> </td>
<td> </td>
</tr>
<tr>
<td>Comeau C++ 4.3.0</td>
<td>X</td>
<td>X</td>
<td>X</td>
<td>X</td>
</tr>
<tr>
<td>GNU GCC 3.3.1</td>
<td>X</td>
<td>X</td>
<td>X</td>
<td>X</td>
</tr>
<tr>
<td>GNU GCC 2.95.3</td>
<td>X</td>
<td>X</td>
<td> </td>
<td>X</td>
</tr>
<tr>
<td>Intel C++ 7.0</td>
<td>X</td>
<td> </td>
<td>X</td>
<td>X</td>
</tr>
<tr>
<td>Metrowerks CodeWarrior 8.3</td>
<td>X</td>
<td> </td>
<td>X</td>
<td>X</td>
</tr>
<tr>
<td>Microsoft Visual C++ 7.1</td>
<td>X</td>
<td>X</td>
<td>X</td>
<td>X</td>
</tr>
<tr>
<td>Microsoft Visual C++ 6 SP5 and 7</td>
<td>X</td>
<td> </td>
<td> </td>
<td> </td>
</tr>
</tbody>
</table></div>
<p>

  </p>
<p>Finally, the current state of the testsuite in CVS may be found on the
    <a href="http://boost.sourceforge.net/regression-logs" target="_top">Test Summary</a>
    page. Please note, however, that this page reports on day-to-day changes
    to inter-release code found in the Boost CVS and thus likely does not
    match the state of code found in Boost releases.</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="variant.troubleshooting"></a>Troubleshooting</h3></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="misc.html#variant.troubleshooting.template-depth">"Template instantiation depth exceeds maximum"</a></span></dt>
<dt><span class="section"><a href="misc.html#variant.troubleshooting.compiler-memory">"Internal heap limit reached"</a></span></dt>
</dl></div>
<p>Due to the heavy use of templates in the implementation of
  <code class="computeroutput">variant</code>, it is not uncommon when compiling to encounter
  problems related to template instantiaton depth, compiler memory, etc. This
  section attempts to provide advice to common problems experienced on several
  popular compilers.</p>
<p>(This section is still in progress, with additional advice/feedback
  welcome. Please post to the Boost-Users list with any useful experiences you
  may have.)</p>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="variant.troubleshooting.template-depth"></a>"Template instantiation depth exceeds maximum"</h4></div></div></div>
<div class="toc"><dl class="toc"><dt><span class="section"><a href="misc.html#variant.troubleshooting.template-depth.gcc">GNU GCC</a></span></dt></dl></div>
<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="variant.troubleshooting.template-depth.gcc"></a>GNU GCC</h5></div></div></div>
<p>The compiler option
        <code class="computeroutput">-ftemplate-depth-<span class="emphasis"><em>NN</em></span></code> can increase the
        maximum allowed instantiation depth. (Try
        <code class="computeroutput">-ftemplate-depth-50</code>.)</p>
</div>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="variant.troubleshooting.compiler-memory"></a>"Internal heap limit reached"</h4></div></div></div>
<div class="toc"><dl class="toc"><dt><span class="section"><a href="misc.html#variant.troubleshooting.compiler-memory.msvc">Microsoft Visual C++</a></span></dt></dl></div>
<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="variant.troubleshooting.compiler-memory.msvc"></a>Microsoft Visual C++</h5></div></div></div>
<p>The compiler option <code class="computeroutput">/Zm<span class="emphasis"><em>NNN</em></span></code> can
        increase the memory allocation limit. The <code class="computeroutput">NNN</code> is a
        scaling percentage (i.e., <code class="computeroutput">100</code> denotes the default limit).
        (Try <code class="computeroutput">/Zm200</code>.)</p>
</div>
</div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="variant.ack"></a>Acknowledgments</h3></div></div></div>
<p>Eric Friedman and Itay Maman designed the initial submission; Eric was
  the primary implementer.</p>
<p>Eric is also the library maintainer and has expanded upon the initial
  submission -- adding
  <code class="computeroutput"><a class="link" href="../boost/make_recursive_variant.html" title="Class template make_recursive_variant">make_recursive_variant</a></code>,
  <code class="computeroutput"><a class="link" href="../boost/make_variant_over.html" title="Class template make_variant_over">make_variant_over</a></code>, support for
  reference content, etc.</p>
<p>Andrei Alexandrescu's work in
    [<a class="link" href="refs.html#variant.refs.ale01a">Ale01a</a>]
and
    [<a class="link" href="refs.html#variant.refs.ale02">Ale02</a>]
inspired the library's design.</p>
<p>Jeff Garland was the formal review manager.</p>
<p>Douglas Gregor,
Dave Abrahams,
Anthony Williams,
Fernando Cacciola,
Joel de Guzman,
Dirk Schreib,
Brad King,
Giovanni Bajo,
Eugene Gladyshev,
and others provided helpful feedback and suggestions to refine the semantics,
interface, and implementation of the library.</p>
</div>
</div>
<div class="copyright-footer">Copyright © 2002, 2003 Eric Friedman, Itay Maman<br>Copyright © 2014-2025 Antony Polukhin<p>Distributed under the Boost Software License, Version 1.0.
    (See accompanying file <code class="filename">LICENSE_1_0.txt</code> or copy at 
    <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
    </p>
</div>
<hr>
<div class="spirit-nav">
<a accesskey="p" href="design.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../variant.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="refs.html"><img src="../../../doc/src/images/next.png" alt="Next"></a>
</div>
</body>
</html>
